The field of the invention is imaging and in particular imaging in X-ray medical imagery. The invention is more particularly concerned with a method and an apparatus for processing radiological images, and particularly images used for example in a vascular examination by Digital Subtracted Angiography (DSA).
Angiography is a medical imagery technique used for a diagnosis or therapeutic purpose during which a practitioner makes an examination of the blood vessels of a patient using X-rays. This technique more precisely allows for visualization of blood vessels by injecting a contrast product (typically based on iodine) that mixes with the blood and temporarily makes it opaque to X-rays.
The contrast product can be injected either intravenously, or intra-arterially using a catheter inserted into the lumen of the arteries. The introduction of the catheter and guidance as far as the artery of interest enables the practitioner to inject the contrast product only locally. If the examination is more invasive than an intravenous injection, the quantity of the contrast product used is lower and the image quality is better because the display is not disturbed by opacification of nearby arteries and/or veins, and the contrast product is significantly less diluted in the blood.
In angiography, an injected image is an image taken while the contrast product is present in the patient's body. In the opposite sense, a non-injected image is an image taken without use of the contrast product.
Since the practitioner's primary interest is the opacified vessel, it is desirable to eliminate the remainder of the patient's anatomy (in other words background structures such as muscles, bones, etc.), which could hide parts of blood vessels due to absorption of the X-rays. The background structures are typically removed by subtracting from an injected image a non-injected image of the area of interest taken under the same conditions. This non-injected image is then called the mask image and the resulting image is said to be subtracted. All that remains in the resulting image is then the arteries containing the contrast product.
In general, an attempt is made to minimize the dose of X-rays applied to the patient during acquisition of the radiographic images. The result is that images are usually affected by a quantum noise. Moreover, this noise is amplified by the subtraction operation and then becomes even more visible on a uniform background. Noise reduction techniques have been proposed that make use of an intra-image spatial filtering. However, these techniques usually cause blur and smoothing of vessel edges. Furthermore, since this spatial filtering modifies the shape of the noise power spectrum, the use of these techniques tends to generate noise clusters, with a granular aspect that practitioners do not accept.